I forgot how to do these :') please help

Simply 8x squared + 5(7x squared- x) - 2x =

Melissa is purchasing a $160,000 home and her bank is offering her a 30-year mortgage at a 4.9% interest rate. in order to lower her monthly payment, melissa will make a 20% down payment and will purchase 3 points. what will her monthly mortgage payment be? a. $645.98 b. $650.46 c. $662.93 d. $606.69

What is the benefit of converting measures in a ratio to the same unit